1. Field of the Invention
The invention relates to a circuit arrangement for producing current pulses of a given shape in at least one inductive load, more particularly a gradient coil for nuclear magnetic resonance tomography, with the use of a supply voltage.
2. Description of the Related Art
If a current pulse of a given shape and amplitude having a high edge steepness should be produced in an inductive load, the value of the supply voltage required to this end is determined in accordance with that voltage which is required to obtain a current increase or current decrease. In the time periods in which a constant current flows through the coil, on the contrary a considerably smaller voltage is required. Consequently, the problem arises that the supply voltage for the current increase or current decrease must be chosen to be very large.
In the circuit arrangements according to the prior art, in series arrangement with the inductive load mostly series regulators are connected, in general transistors, by means of which the required flow of current is adjusted. The difference between the supply voltage and the voltage at the inductive load in this series regulator decreases and leads to high losses. This holds especially during the time periods in which a constant flow of current should be adjusted, because especially during these time periods the supply voltage itself is considerably larger than is required.
According to the prior art, further a second solution is known, in which the supply voltage either can be changed over to different values or in which a combinatorial circuit part is provided in order to continuously adapt the supply voltage required. However, in both cases, always a sufficient voltage reserve for the series regulators must be ensured. In the solution with different voltages (for example EP 0 250 718 A1) which corresponds to U.S. Pat. No. 4,733,342, many switches and voltage sources are required, which must all be designed for the maximum current, which renders the circuit arrangement much more complicated. Also in circuit arrangements using this solution, still high losses occur.